The present invention relates to a process for producing sintered, microcrystalline bodies based on .alpha.-Al.sub.2 O.sub.3, and to their use.
One preferred application of .alpha.-Al.sub.2 O.sub.3 sintered bodies is their use as abrasive media. In addition to fused corundum abrasive media, such media made of sintered material have already been known for more than 50 years. The usable grades of .alpha.-Al.sub.2 O.sub.3 are derived from ones using reduction andr melting (fusing) methods.
U.S. Pat. No. 3,909,991 describes polycrystalline .alpha.-Al.sub.2 O.sub.3 bodies, the crystallite size of which is in the submicron range and the density of which is greater than 95% of the theoretical density. They are produced by hot-pressing a mixture of carbon black and granulated .alpha.-Al.sub.2 O.sub.3 bodies which are obtained according to U.S. Pat. No. 3,079,243 by comminuting cold-pressed .alpha.-Al.sub.2 O.sub.3 moldings.
More recently, similar sintered abrasive media based on .alpha.-Al.sub.2 O.sub.3 have become known which have advantages compared with the known corundum abrasive media due to their microcrystalline structure. Thus an abrasive medium is disclosed in European patent 0 152 768 which is produced at sintering temperatures of about 1400.degree. C. via the sol-gel technique. Crystallization nuclei are added as sintering aids. Similar processes and materials are disclosed in European Application 0 024 099, German Application 3 219 607, U.S. Pat. No. 4,518,397, U.S. Pat. No. 4,574,003, U.S. Pat. No. 4,623,364, and European Applications 0 168 606, 0 200 487, 0 228 856, 0 209 084 and 0 263 810.
A common feature of all the last-mentioned processes is that they are carried out via a sol-gel process using very finely dispersed alumina monohydrate of the boehmite type. The relatively expensive raw materials, which are obtained via the hydrolysis of organic compounds of aluminum, and the energy-consuming process technology cause the cost of sol-gel corundum to increase to many times that of conventional corundum products. Another disadvantage of this process is that the colloidal solutions are for the most part stabilised with relatively large amounts of readily volatile inorganic acids, which leads to processing and environmental problems.
German Patent 3 604 848 describes a process comprising the comminution of a dispersion of alumina-containing raw materials, compounds containing hydrated silica, and other additives (e.g. compounds of the metals Co, Ni, Mg, Cr, Zr, Zn, Si or Ti) to produce a sinterable slip, from which an abrasive medium, the corundum primary crystallites of which have a diameter less than 5 microns, can be produced by drying in stages and sintering at temperatures up to 1700.degree. C. The product obtained in this manner, with its crystallite size of less than 5 microns, still does not have the microstructure of a material which is produced via the sol-gel process using appropriate sintering additives.
Furthermore, compounds containing hydrated silica and which function as sintering aids have to be added according to the process disclosed in German Patent 3 604 848. Mullite is formed from these added compounds on sintering due to their reaction with the alumina. It is known that the effective performance of an abrasive grit is reduced by the presence of silicate phases.
In recent years, numerous abrasive technology investigations on sintered corundum have shown that grinding capacity is inversely proportional to the size of the primary crystals. This means that the finer the microstructure is, the higher the grinding capacity is as a rule.
European Application 0 524 436 discloses a process in which other, inexpensive precursors of alumina (e.g. hydrargillite) are used instead of the expensive boehmite. Suspensions with a solids content between 10 and 40% by weight, which can be processed further analogously to the sol-gel process, are obtained by comminution and subsequent deagglomeration. The residual water also has to be removed in this process, with a very high energy consumption. With the exception of the expensive raw material, the process described in European Application 0 524 436 has all the process technology disadvantages of the sol-gel method.
Compared with the sol-gel process, however, all previously known alternative processes for producing microcrystalline sintered corundum products which start from inexpensive raw materials have the disadvantage that structures are present after sintering which are significantly coarser than those of products obtained via the sol-gel process. The reasons for this are obvious. Sol-gel processes start from particularly finely-divided starting materials which, with the addition of sintering aids, can be dense-sintered at very low temperatures, due to the process employed. Crystal growth is thereby suppressed. In principle, the alternative processes necessitate higher sintering temperatures, which lead to increased crystal growth. Moreover, inhomogeneous and uncontrolled crystal growth is initiated by the phase transformations to .alpha.-Al.sub.2 O.sub.3 which occur on sintering. The effective performance of sintered corundum obtained via the alternative processes is thus considerably less than that of sol-gel corundum products.
European Application 0 402 686 discloses a process for obtaining microcrystalline corundum via electrophoretic deposition from an organic suspension containing .alpha.-Al.sub.2 O.sub.3. A relatively dense green body can be obtained in this manner, which, despite its dense, homogeneous packing, however, can only be dense-sintered at temperatures of 1600.degree. C., so that this process also results in increased crystal growth. The structure of sintered corundum obtained via electrophoretic deposition is considerably coarser than that of sintered corundum products obtained via the sol-gel process. The effective performance of sintered corundum obtained via electrophoresis is also correspondingly less.
The object of the present invention is thus to demonstrate a process for producing a microcrystalline .alpha.-Al.sub.2 O.sub.3 sintered body which does not have the disadvantages of the prior art described above.